Well I don't have a exhaust drone. Since the car is getting older, there is sometimes weird noises like creeks and such. I was also thinking that the extra 1/8-1/4" thickness of it might also make it alot harder to put everything back together.

I didnt think it was that thick. I doubt very may areas would give you trouble with the added thickness but there are tricks like stopping short and notching around areas. You could also just lay some in areas and get a difference. My guess along with a quieter ride it may give a slight insulation value as well making car easier to heat and cool. Its very popular for stereo installs as it helps the interior of car sound better too. Stereo installation shops could proba ly give some tips too. I think there are paint on type barriors now too.